There are two components of working memory (WM): maintenance of information for a short period of time and manipulation of information necessary for higher cognitive functions, such as language, reasoning and problem solving. Earlier WM studies have demonstrated that dorsolateral prefrontal cortex (DLPFC) activity increases as maintenance and load demand increases. Suggesting a role of DLPFC in the active maintenance of information. Few studies have carefully disambiguated these two WM processes at the behavioral and physiological levels. Using functional magnetic resonance imaging (fMRI) and a block design of the Sternberg task, we mapped within the prefrontal cortex (PFC) locales that are selectively load sensitive, delay sensitive, or both. In considering the Sternberg task, WM load was manipulated by varying the memory set size (3, 5, or 8 letters) and the delay period between the string of letters and probe. The effect of memory maintenance was examined by employing two time delays between the letter set and probe stimuli. We found the DLPFC was strongly activated in load manipulation, whereas activation as a function of delay was restricted to the left premotor regions and Brocas areas. Regions of PFC on the right were found to be exclusively affected by load. These results suggest the possibility that top-down modulation of attention or cognitive control at encoding and/or decision making may be mediated by these areas.